The present invention relates to a magnetic recording and reproducing apparatus, for example, a video tape recorder of the rotary two-head type which records a video signal in the direction of width of a magnetic tape by means of two rotary magnetic heads and which reproduces this recorded signal by the same heads and, more particularly, to an improvement of a circuit for producing a signal by which the above two magnetic heads are alternately switched, thereby enabling the recording/reproducing of the video signal.
FIG. 1 is a block diagram illustrating a conventional head switching signal producing circuit in a video tape recorder of the rotary 2-head type. FIG. 2 is a diagram showing the output waveforms at the main parts of the circuit of FIG. 1.
In FIG. 1, reference numerals 10 and 20 denote magnetic heads for alternately recording or reproducing a video signal on or from a magnetic tape 4 for every field. These two magnetic heads 10 and 20 are attached on a disk 3 at an angle of 180.degree. apart from each other and are rotated at a constant speed by a disk motor 2 together with the disk 3. Two magnets 11 and 21 are respectively attached to the disk 3 at an angle of 180.degree. apart from each other with respect to the positions of the magnetic heads 10 and 20 and these are detected by a tack head 1. A signal H.sub.1 indicative of the position which was detected with respect to the magnet 11 and a signal H.sub.2 representing the position which was detected with respect to the magnet 21 are respectively separated from an output H from the tack head 1 by a separating amplifier 5 and are output. The output signals H.sub.1 and H.sub.2 from the circuit 5 are respectively supplied to delay circuits (e.g. multivibrators) 6 and 7 and are individually delayed, respectively. A reference numeral 8 indicates an R/S flip flop circuit; its output Q is triggered (set) by an output A.sub.1 from the delay circuit 6 and becomes a high level "H", while it is triggered (reset) by an output A.sub.2 from the delay circuit 7 and becomes a low level "L". The output Q from this circuit 8 is output as a head switching signal from a terminal 100. The phase of the magnetic head 10 is specified by the phase of the leading edge of this head switching signal Q, while the phase of the magnetic head 20 is specified by the phase of the trailing edge of the signal Q. However, since the positions of the two magnetic heads are detected through the magnets as mentioned above, it is necessary to adjust the phase of the signal Q in accordance with the relative attaching positions of the magnetic heads 10, 20, magnets 11, 21, and tack head 1. Due to this, conventionally, the phases were needed to be individually adjusted by the delay circuits 6 and 7, respectively.
In addition, the above-mentioned head switching signal Q is used as a reference signal of the head disk servo system for correctly recording a video signal in the specified position of the magnetic tape 4 in the recording mode, while it is used to correctly switch the video signal to be alternately reproduced for every field by the magnetic heads 10 and 20 in the reproducing mode. Therefore, if the above phase adjustment is incomplete, the normal recording and reproducing will not be performed, so that this causes problems such that noises appear in the reproduced picture and that the compatible reproduction becomes difficult, and the like. Due to this, it is necessary to respectively individually and finely adjust in such a manner that in the delay circuits 6 and 7, the spatial relative positions of the magnetic heads 10 and 20 and the magnetic tape 4 have the specified relationship and that the duty ratio of the head switching signal Q is accurately 50%. Thus, there is a problem of increase in total cost of a set since it takes a longer time for adjustment in addition to complication of its adjusting procedure. In FIG. 2, V is a false sync signal and B.sub.1 and B.sub.2 are output signals of the delay circuits 6 and 7.